(Source: SL Market Letter # 223)

Download  this essay (in .zip format) for offline reading

THE FIRST CARS SOLD to the public with fuel injection were Mercedes-Benz 260D (diesels) in 1936. Gasoline applications were more difficult, though M-B's Racing Department was planning its introduction for the 1940 Grand Prix racing season. The outbreak of WWII. circumvented M-B's 1940 participation. Within three years the technology surfaced in both German and U.S. combat aircraft.

The idea of precisely metered gas/air ratios at each cylinder was attractive, as carburetors were infamous for uneven distribution. A big reason for multiple carburetors (six of them on some V-12 Ferraris) was not so much for volume of output (though it helped) as to bring the delivery systems closer to their assigned cylinders. For cars Bosch had to smooth out low R.P.M. fuel delivery (not so critical in airplanes where 2/3 throttle or more was the norm). Automatic adjustments for altitude, humidity & temperature changes were a bonus inherited from planes.

Germany's Goliath and Gutbrod minicars were the first to sell the Bosch units on gas engines, (2 cylinder, 2 stroke) in 1951, though public interest in two stoke engines was on the wane. Both cars were short lived. The M-B racing department encouraged its development over supercharging as the power boost was from more precise fuel distribution not just more air & fuel. The ideal air/gas ratio, 13.7:1, was produced without wasting fuel or the opposite risk, too much air, which can quickly burn valves. Fast race cars seldom exceeded 3 to 4 M.P.G. & fuel injection double these figures! In street form, production 300SL Gullwings frequently hit 20 M.P.G., while Ferrari of the same period give 10 M.P.G. Throttle response was also faster, as injectors were in or closer to each cylinder than possible with carburetors.

Variations were tested, including a combination of fuel injection and supercharging (which M-B reincarnated for the 1996 SLK). One factory prototype 190SL was built with fuel injection, but the cost/benefit ratio was not justified given the inherent breathing limitations of that engine.

No less than 57 changes occurred during 17 years. Three big ones were:

a) moving the injection nozzles into the intake portion of the cylinder head (some had tentacles protruding way into the head from the intake manifold).

b) switching from vacuum to mechanically activated fuel metering.

c) reducing plungers to 2, 1 for each 3 cylinders. This may have been inspired by GM's Rochester fuel injection of 1957 that only had one plunger for eight cylinders. Lance Reventlow's Scarabs won many races with this simple system, but when it clogged from any foreign particles the whole motor stumbled, rather than just one (or three) cylinders. Mercedes did not make this change on all models, and returned (retreated?) to a plunger for each cylinder in 1963 & 64. The 230/250/280SL always had six plunger mechanic pumps, and the 6.3 V-8s through 1972 retained eight plunger mechanical pumps. Both used manifold mounted injectors.

Experiments with electronic fuel injection did not satisfy M-B engineers enough for it to reach production until the 3.5 V8's. This required new tuning and rebuild equipment & knowledge. By 1975 catalytic convertors added more demands. In 1976 a mechanically driven continuous injection system (CIS) was introduced. These models will be discussed at another time.

CARE AND FEEDING OF MECHANICAL FUEL INJECTION PUMPS

Considering what they do (see shaded area on page 5) Bosch pumps are extremely reliable. We asked our favorite authorized rebuilder, Robert Fairchield (Jerry Fairchield Industries, Redding, CA 916-241-1592) what advice he had for owners:FAIRCHIELD: "These mechanical pumps are vastly superior to carburetors. 200,000 miles without repairs is not unusual, & when pumps are rebuilt they often require little more than a good cleaning and new "0" rings. If they get proper lubrication wear is minimal."

SLML: What can owners do to prolong the life of their pumps?

FAIRCHIELD: "Monitoring the correct level of oil is important. There are two portions of the pumps that receive lubrication... the lower portion has its own fillable oil reservoir. All except the 280SL have a dipstick that can be checked. Too much oil is as bad as too little. Lack of lubrication is obvious too much inhibits proper advancement of the flywheels that are in the lower portion... and this allows the pump to run too rich. Overfilling on 220, 230 & 250 engines can be caused by careless servicing, or by leaky "0" rings from the plunger chambers up above. The upper parts receive lubrication separately. Leaky "0" rings will cause flooding of the lower pump chamber (in 280SLs overflow drains back into the engine). In addition to poor M.P.G. running too rich washes oil off cylinder walls (maximizing wear) and diluting oil (destroying main bearings). Maximum permissible oil dilution is 5 percent."

SLML:What advice do you have for people that store their favorite cars in the winter or longer?

FAIRCHIELD: "Storage causes more problems than regular use. The big enemies are dry gas residues plus debris & rust from old gas tanks & lines. The best line of defense is before storage. Add a can of Sta-Bil to that last gas fill before storage, & avoid butane additives that are added (around November) in some states to help winter engine starting. Once particles get into the injection pump it's counterproductive for owners to attempt much flushing. Fuel injection cleaners may dissolve & wash out dried gas, but they also loosen pieces of rust & debris in the gas tank & move it forward into the pump! That blockage is rarely removed without getting inside the pump & gas tank"

SLML:What significant changes have occurred in pumps from 1954 to 1972?

FAIRCHIELD: "The 300SL used a vacuum governor with replaceable diaphragm. Starting with the 230SL the governor became mechanical... no diaphragm. 280SLs eliminated separate lubrication of the lower chamber... the whole pump is lubricated by the engine with a return drain back to the engine preventing overflow risks."

SLML: Is it possible to switch units from one Mercedes model to another?

FAIRCHIELD: "It can be done if the correct governor is transplanted to the replacement unit. The fuel cams in the pumps need to be switched as fuel flow requirements vary. I once converted a 300d sedan pump for a 300SL that Scott Restoration was rebuilding."

SLML: Do you have any standardized fees or is every pump repair different?

FAIRCHIELD: "I'm able to do most rebuilds with recalibration for $350 to $475 plus parts. Turnaround time is 4 to 10 days. Parts are expensive, but in my experience broken parts aren't the rule. Flyweights can cost $700. Cam shafts $250. Plungers & barrels are $250 each. I am not averse to using a customer's spare parts if they've located a salvage pump. Buying a few correct spares now, including a pump, might become mighty wise in 10 years."

SLML:Do you get many requests to restore the exterior of pumps?

FAIRCHIELD: "More each year. I'm able to re-cadmium plate the exterior parts before show type assembly for $100 to $200 depending on model & deterioration."

We also talked with John Mashinter, (Mashinter Research 1-800-887-0671) an authorized Bosch rebuilder in Rockford, IL. His father, William (deceased), who started their company, was involved in developing early U.S. aircraft fuel injection. Bill wrote the following 30 years ago:

WHAT FUEL INJECTION DOES

1. Consider that there are 231 cubic inches in a gallon of gasoline & 16,400 cubic millimeters(mm) in a single cubic inch. Modern injection systems meter as less than 10 cubic mm per explosion. This is 1/1640 cubic inches.

2. The small quantity per explosion must be injected over a wide speed range. At 6,400 R.P.M. an injection pump delivers 19,200 individual fuel charges per minute.

3. The individual fuel charges are delivered over a very small number of crankshaft degrees at an exact timed relationship to the engine cycle.

4. The individual charges are delivered over a wide range of ambient and operating temperatures. 5. The individual charges are delivered through an injection valve at 600 psi.

6. The quantities of fuel per injection are also controlled so that there will be very little variation between the fuel discharged at individual cylinders.

7. The injection valve is subjected to 1200 degree combustion temperatures on one end and variable outside temperatures on its other end.

8. The above task must be satisfactorily accomplished over thousands of miles at the direction of an operator who is usually technically ignorant of the nature of task he's directing.

The wonder of it all is not that the system is so reliable, but rather that it works at all.

William Mashinter - 1968 [1990s Mercedes-Benz refinements also adjust for type & quality of gas, prevailing compression at each cylinder and type of driving]

SLML:Have you noticed any changes in the type or quantity of "old-timer" customers during the 1990s?

MASHINTER: "There's growing interest in 1960s models. I get more 230,250, & 280SL pumps than my father did. There were more SLs sold in the 60s, but love for these models is definitely kindling."

SLML:What advice do you have for owners trying to maintain their pumps?

MASHINTER: “I tell clients to add a little 2 stroke oil (outboard motor type oil) to their gas every third or fourth tank full.  This is preferable to automatic transmission fluid [recommended elsewhere] as it is made to be burned as it lubricates.  Fuel line cleaners create more debris than they can remove.” 

Further Reference: An excellent 18 page report on these pumps will be found in The Mercedes Collector, Issue 6, 3/93. $10 per back issue. Check only, c/o P.O. Box 4181, Silver Springs, MD 20904. 

Return to the Sample Articles Index